1. Field of the Invention
The present invention relates to combustor and, especially, a gas turbine combustor.
2. Description of the Related Art
A combustor has been used in various fields. The need for combustion at a high air-fuel ratio, i.e., a lean-burn combustion has increased as the exhaust emission, especially, the exhaust emission of NOx has become strictly regulated. A fluctuation in combustion tends to occur as lean-burn combustion takes place, this resulting in a fluctuation in the pressure of a combustion gas.
For example, as shown in FIG. 10, in a gas turbine, a casing 100 covers, but is separate from, a plurality of combustors 200 in which combustion takes place. An intake chamber 300 is formed between the combustors and the casing. The air discharged from a compressor is introduced into the intake chamber 300, and into the inside of the combustors 200, and is mixed with fuel supplied from fuel nozzles 400 to burn. Thus, the combustion gas is introduced into a turbine portion.
The intake chamber 300 is generally annular, and is very large, i.e., the length thereof in the axial direction is often more than 2 m and the width thereof in the radial direction of the annulus is often more than 1 m. This large intake chamber forms a sound field and, accordingly, if the pressure in the combustors 200 varies due to the combustion fluctuation, the pressure variation is transmitted to the intake chamber 300, so that a frequency component corresponding to a natural frequency of the sound field is amplified and re-propagated to the combustors 200. Accordingly, the pressure variation in the combustors 200 is further increased. Consequently, a so-called combustion vibration phenomenon occurs, in which the amount of fuel or air introduced into the combustors varies and the burning fluctuation is further enhanced.
Japanese Unexamined Patent Publication (Kokai) No. 11-62549 discloses an acoustic material, or sound absorbing material, attached to the inner wall of the casing 100 to restrict the air-vibration-amplifying operation in the intake chamber 300.
However, the intake chamber 300 is subject to severe conditions, i.e., 500° C. in temperature and 2.5 MPA in pressure, and is positioned on the upstream side of a turbine chamber which rotates at high speed. It is required that the acoustic material cannot be broken or scattered even under the above severe conditions. In fact, it is very difficult to obtain an acoustic material which meets the above requirements at a reasonable cost.